Evaporation reduction from reservoirs Which chemical is used to form a monomolecular surface film to minimize evaporation losses from reservoirs?

Introduction / Context:
In arid and semi-arid regions, open-water evaporation from reservoirs is a significant loss. One mitigation technique is spreading a monomolecular film of long-chain fatty alcohols on the surface to reduce evaporation by lowering surface tension and suppressing capillary waves.

Given Data / Assumptions:

• Objective is evaporation reduction, not disinfection or pH modification.
• Candidates include chemicals with low volatility and ability to spread as a thin film.

Concept / Approach:
Cetyl alcohol (hexadecanol) and stearyl alcohol (octadecanol) are classic film-forming agents used to reduce evaporation. They are hydrophobic, spread spontaneously into a monolayer, and are effective under calm to moderate conditions. Acids, methane, or simple alcohol spirits are unsuitable for sustained monolayer film formation.

Step-by-Step Solution:

Verification / Alternative check:
Engineering literature documents successful trials using cetyl or stearyl alcohol, sometimes in blends, to reduce evaporation rates.

Why Other Options Are Wrong:
Industrial spirit evaporates quickly and does not form durable films; hydrochloric acid and methane are unsafe/irrelevant; “None of these” is incorrect because an effective agent (cetyl alcohol) exists.

Common Pitfalls:
Confusing “acetyl” with “cetyl”: the effective film former is cetyl alcohol (C16H34O), not acetyl compounds.

Final Answer:
Cetyl alcohol (hexadecanol)